Although aluminum protects itself against corrosion by forming a natural oxide coating, the protection is not complete. In the presence of moisture and electrolytes, aluminum alloys corrode much more rapidly than pure aluminum.
Accordingly, there is a need to treat aluminum alloy substrates with primers or other chemicals that provide improved corrosion resistance as well as strong bonding affinity for polymers.
In the prior art, chemical conversion coatings have been formed on aluminum alloys by "converting" a surface of the metal into a tightly adherent coating, part of which consists of an oxidized form of aluminum. Chemical conversion coatings provide high corrosion resistance and improved bonding affinity for polymer coatings. A chromate conversion coating is typically provided by contacting aluminum with an aqueous solution containing hexavalent or trivalent chromium ions, phosphate ions and fluoride ions. In recent years, concerns have arisen regarding the pollution effects of chromates and phosphates discharged into waterways by such processes. Because of the high solubility and strongly oxidizing character of hexavalent chromium ions, expensive waste treatment procedures must be employed to reduce the hexavalent chromium ions to trivalent chromium ions for waste disposal.
Attempts have been made in the prior art to produce acceptable chromate-free conversion coatings for aluminum. For example, some chromate-free conversion coatings contain zirconium, titanium, hafnium and/or silicon, sometimes combined with fluorides, surfactants and polymers such as polyacrylic acid. In spite of the extensive efforts that have been made previously, there is still no entirely satisfactory non-chromate conversion coating or primer for improving the adhesion and corrosion resistance of polymer coated aluminum alloy substrates.
A principal objective of our invention is to provide polymer coated aluminum alloy sheet having a primer layer comprising a reaction product of a vinylphosphonic acid-acrylic acid copolymer and an aluminum oxide or hydroxide layer on the sheet.
A related objective of our invention is to provide a primer layer for aluminum alloy sheet that is free of chromium compounds.
Additional objectives and advantages of our invention will become apparent to persons skilled in the art from the following detailed description.